Core Concept

Heat flow (also called heat transfer) refers to the movement of thermal energy from a region of higher temperature to a region of lower temperature. Heat flow plays a critical role in chemistry, affecting reaction rates, state changes, and chemical equilibria.

Key Concepts

1. Direction of Heat Flow:

   • Heat flows from a hotter object to a colder object until thermal equilibrium (equal temperature) is reached.

   • Endothermic processes absorb heat from the surroundings (e.g., melting, vaporization).

   • Exothermic processes release heat to the surroundings (e.g., freezing, combustion).

2. Units of Heat:

   • Joules (J): The SI unit of energy and heat.

   • Calories (cal): Often used in food energy, with 1 cal = 4.184 J.

   • Kilocalories (kcal): 1 kcal = 1000 cal and is often labeled as “Calories” in food context.

Types of Heat Flow Processes

1. Calorimetry:

   • Calorimetry is the study t of heat involved in a chemical reaction or phase change. See calorimetry page.

2. Phase Changes and Heat Flow:

   • During a phase change (e.g., melting, boiling, freezing), heat is absorbed or released without a temperature change.

   • Heat of Fusion ($\Delta H_{\text{fus}}$​): Heat absorbed when 1 mole of a substance melts.

   • Heat of Vaporization ($\Delta H_{\text{vap}}$​): Heat absorbed when 1 mole of a substance vaporizes.

   • Formula: q = n⋅ΔH

   • Where:

     • q: Heat (in joules or calories)

     • n: Moles of the substance

     • ΔH: Enthalpy change for the phase transition (fusion or vaporization)

3. Enthalpy (ΔH):

   • Enthalpy is the heat content of a system at constant pressure.

   • Enthalpy of Reaction ($\Delta H_{\text{rxn}}$​): The heat absorbed or released during a chemical reaction.

     • Exothermic reactions have negative ΔH (heat is released).

     • Endothermic reactions have positive ΔH (heat is absorbed).